1. Field of the Invention
The present invention relates to a holding seal material, a manufacturing method for a holding seal material, an exhaust gas purification apparatus and a manufacturing method for an exhaust gas purification apparatus.
2. Background Art
Exhaust gas emitted from an internal-combustion engine such as a diesel engine contains particulate matter (hereinafter, also referred to as PM), and, in recent years, the harmful influence of the PM on the environment or human bodies has become an issue. In addition, exhaust gas also contains harmful gas components such as CO, HC or NOX, and thus there is another concern regarding the influence of the harmful gas components on the environment or human bodies.
Therefore, a variety of exhaust gas purification apparatuses made up of an exhaust gas treatment unit made of a porous ceramic such as silicon carbide or cordierite, a metal casing housing the exhaust gas treatment unit, and a holding seal material made of an inorganic fiber and disposed between the exhaust gas treatment unit and the metal casing have been proposed as an exhaust gas purification apparatus that traps PM in exhaust gas or purifies the harmful gas components. The holding seal material is disposed mainly for preventing the exhaust gas treatment unit from coming into contact with the metal casing covering the outer circumference of the exhaust gas treatment unit so as not to be broken due to oscillation or impact generated during the running or the like of a vehicle or for preventing exhaust gas from leaking from between the exhaust gas treatment unit and the metal casing.
Since an internal-combustion engine is operated under a condition approximate to the theoretical air fuel ratio for the purpose of an improvement of gas mileage, there is a tendency for exhaust gas to have a high temperature and a high pressure. When the holding seal material is exposed to exhaust gas having a high temperature and a high pressure, there is a case in which the inorganic fiber constituting the holding seal material deteriorates due to heat. Therefore, the holding seal material is required to have excellent thermal resistance. In addition, when exhaust gas having a high temperature and a high pressure arrives at the exhaust gas purification apparatus, the space between the exhaust gas treatment unit and the metal casing may change due to the difference in thermal expansion coefficient between the exhaust gas treatment unit and the metal casing. Therefore, the holding seal material is required to have a strong holding force (expanding pressure) enough to sufficiently hold the exhaust gas treatment unit even when the space has been changed. Furthermore, the holding seal material is required to have a sufficient sealing property to prevent exhaust gas from leaking between the exhaust gas treatment unit and the metal casing.
In addition, since the holding seal material is made of an inorganic fiber, the holding seal material contains a large amount of a stimulant fine inorganic fiber (for example, a fiber having a diameter in a range of approximately 3 μm to 8 μm). When an exhaust gas treatment unit is produced using the above-described holding seal material, there is a problem in that the inorganic fiber scatters around from the holding seal material when an operator handles the holding seal material, and the operational environment deteriorates.
Furthermore, when the holding seal material is wound around the exhaust gas treatment unit, there is a case in which fissures are generated on the outer surface of the holding seal material. The fine inorganic fiber, sometimes, scatters through the fissures.
Japanese Unexamined Patent Application Publication No. 2003-129832 discloses a bilayer-structured holding seal material made up of a mat disposed on an exhaust gas treatment unit side and made of a ceramic fiber and a mat disposed on a metal casing side and made of a ceramic fiber.
In the holding seal material disclosed in Japanese Unexamined Patent Application Publication No. 2003-129832, a mat having excellent thermal resistance is disposed on the exhaust gas treatment unit side that reaches a high temperature, and a mat having excellent flexibility is disposed on the metal casing side, thereby preventing the deformation and quality deterioration of the holding seal material and improving the adhesion with the exhaust gas treatment unit or the metal casing.
In addition, in the holding seal material disclosed in Japanese Unexamined Patent Application Publication No. 2003-129832, the longitudinal-direction length of the mat disposed on the exhaust gas treatment unit side having a small curvature radius is made to be shorter than the longitudinal-direction length of the mat disposed on the metal casing side having a large curvature radius, thereby removing a gap in an engagement section when the holding seal material is wound around the exhaust gas treatment unit and improving the sealing property.
Furthermore, in the holding seal material disclosed in Japanese Unexamined Patent Application Publication No. 2003-129832, the holding seal material is sealed with an airtight sheet, thereby preventing the damage of the ceramic fiber and facilitating the attachment to the metal casing.
Japanese Unexamined Patent Application Publication No. 2010-101308 discloses a holding seal material housed in an inside space of a packaging material to decrease the scattering of the inorganic fiber from the holding seal material.
Japanese Unexamined Patent Application Publication No. 2001-521847 discloses a holding seal material having a flexible sheet or coating attached to the outside surface to prevent the generation of fissures on the outside surface of the holding seal material when the holding seal material is attached to the exhaust gas treatment unit.